There is growing interest in using non-woody species, such as wheat straw and hemp, for pulping and papermaking. Economically, these materials can find value-added utilization that would enhance the profitability of farm production.
As future worldwide fiber shortages are predicted, agricultural fibers are believed to be a sustainable fiber supply to potentially substitute wood fibers in certain paper applications. On the other hand, market forces and, perhaps, legislative requirements may stimulate the production of an "environmentally friendly" paper that contains agricultural fibers, as exemplified by the recent experience with recycled fibers.
The art of papermaking was originally developed using non-wood plant sources, including wheat straw, and the production of pulp and paper from wood is a relatively recent development. Pulping processes can be broadly divided into two large categories: chemical pulping and mechanical pulping. The chemical pulping involves using chemical reactions to solubilize lignin and produce individual fibers or pulp from lignocellulosic raw materials. Within the mechanical pulping, there are many processes which involve varying combinations of chemical, mechanical and thermal treatments to effect fiber separation, remove some lignin and other chemical components from the original fibers, or increase the brightness or papermaking strength of the resulting fibers.
One of the problems associated with the chemical pulping of straw is its heavy environmental impact because of a high silica content of the fibers, inherent in most agricultural residues, which makes conventional chemical recovery difficult. Alternatively, mechanical pulping seems to be suitable for cereal straws (wheat, oat, barley, rice), particularly wheat straw, since the latter is easy to disintegrate by mechanical action. Mechanical pulping generates a minimal volume of effluent, thus reducing the environmental impact.
Chemimechanical pulps (CMP) from wood are produced by processes in which roundwood or chips are treated with weak solutions of pulping chemicals such as sulfur dioxide, sodium sulfite, sodium bisulfite or sodium hydrosulfite, followed by mechanical defibration.
Alkaline peroxide mechanical pulping (APMP) is one of the processes to consider to produce bleachable pulp for printing grade papers using non-woody species, such as straw and hemp, as raw material. In U.S. Pat. Nos. 4,849,053 and 5,002,635, Gentile et al. propose that a wood pulp of improved quality is produced from chips using pretreatment with stabilizers and alkaline peroxide prior to refining. The APMP process is based on the incorporation of peroxide bleaching into chemical impregnation and refining stages in which bleaching action takes place not only to eliminate alkali darkening of wood chips but to brighten them to certain brightness levels as well. Therefore, it allows the production of a fully bleached pulp with no need to install a separate bleaching plant (Cort, C. J. and Bohn, W. L., "Alkaline Peroxide Mechanical Pulping of Hardwoods", Tappi J., 74(6): 79-84, 1991). Like sulfonation, carboxylation of lignin by alkaline peroxide results in easier fiber separation during refining and improved fiber bonding in papermaking. Due to its suitability for low-density hardwoods (Cort et al, supra), adaptation of the wood APMP process to straw and hemp appears obvious. The process is environmentally friendly, high-yielding, and uses non-sulfur pulping and chlorine-free bleaching. The alkaline peroxide impregnation stage of the APMP process is similar to conventional bleaching in many respects.
Various pulping and bleaching processes are described in the following patent literature: WO 96/25552 (Henricson et al.), U.S. Pat. No. 4,793,898, WO 94/006964 (Chang et al.), WO 86/05529 (Laamanen et al.), WO 94/17239 (Nilsson et al.), WO 94/29515 (Tibbling et al.) and U.S. Pat. No. 4,400,237.
U.S. Pat. No. 5,320,710 discloses a soft high strength tissue using long-low coarseness hesperaloe fibers. A significant challenge to the papermaker is to make tissues which are not only soft, absorbent and thick but also strong. Typically, softness, absorbency, and thickness are inversely related to strength. High strength specialty papers have been made using non-woody fibers usually termed hard or cordage fibers, such as sisal, abaca, hemp, flax and kenaf. As described in Mclaughlin and Schuck, Econ. Bot 45 (4), pp. 480-486, 1991, such fibers are commonly used for such products as currency paper, bank notes, tea bags, rope paper, filters, air cleaners and other products requiring scruff and tear resistance along with high endurance for folding.
U.S. Pat. No. 4,106,979 discloses a method for the preparation of paper pulps from dicotyledonous plants, such as kenaf and hemp. A dicotyledonous plant has two morphologically distinctive regions in its stem, the outer or bark fraction which contains the bast fibers and the inner or woody core fraction.
Hydrogen peroxide is a versatile and widely used bleaching agent in the pulp and paper industry. It can be used to increase the brightness of mechanical pulps and to delignify and brighten chemical pulps in a multi-stage bleaching sequence. It is generally accepted that hydroperoxide anion is the principal active species in peroxide bleaching systems. As its formation can be regulated by pH, the alkalinity of the bleach liquor should be high enough to ensure an adequate concentration of hydroperoxide anion.
On the other hand, hydrogen peroxide is unstable in alkaline conditions and readily decomposes. The decomposition is accelerated by increasing pH and temperature and the presence of certain transition metals, particularly iron, copper and manganese. This metal-catalyzed decomposition of hydrogen peroxide is generally considered undesirable in the bleaching operation since it leads to a loss of brightening capacity. Additionally, the decomposition products include molecular oxygen, hydroxyl radical (HO.sup.-) and superoxide anion radical (O.sub.2.sup.-), and they may participate in degradation reactions of both lignin and carbohydrates and in chromophore-creating reactions.
In the hemp and wheat straw APMP process, it is critical to produce a pulp of high brightness without significant loss of pulp yield. To meet this requirement, one must fully utilize the brightening potential of hydrogen peroxide and minimize its nonfunctioning loss. As mentioned above, the decomposition of hydrogen peroxide under alkaline conditions is greatly influenced by the presence of certain inorganic compounds i.e. transition metal ions. Conversely, alkali-earth metals like magnesium and calcium, as well as silicon, are considered peroxide stabilizers. To control peroxide decomposition, a proper balance should be sought between these two categories of metals. While all these metals are either initially present in fiber raw materials or introduced as impurities from the bleaching chemicals, process water and equipment, removing or deactivating the transition metals is essential to minimizing the occurrence of catalytic peroxide decomposition. In practice, two approaches, commonly used together, are employed to achieve the pretreatment of pulp before bleaching and stabilization of bleach liquor. Chelation is an effective way to complex and wash out metals from pulp using chelating agents such as diethylene triaminepenta-acetic acid (DTPA) and ethylene diaminetetra-acetic acid (EDTA). See U.S. Pat. Nos. 4,849,053, 5,002,635 to Gentile et al. and U.S. Pat. No. 4,732,650. As a second approach, sodium silicate and magnesium salts have proven stabilizing effects and are in widespread use (Ali, T. et al, "The Roles of Silicate in Peroxide Brightening of Mechanical Pulp 1. The Effect of Alkalinity, pH, Pre-treatment with Chelating Agents and Consistency", J. Pulp Paper Sci., 12 (6): J166-J172 (1986), and Colodette, J. L. et al, "Factors Affecting Hydrogen Peroxide Stability in the Brightening of Mechanical and Chemimechanical Pulps. Part 111: Hydrogen Peroxide Stability in the Presence of Magnesium and Combinations of Stabilizers", J. Pulp Paper Sci., 15 (2): J45-J50 (1989).
In addition, chelating agents such as DTPA and diethylene triaminepentamethylene phosphonic acid (DTPMPA) are also used as organic stabilizers for bleach liquor stabilization (U.S. Pat. No. 4,732,650 and Kuczynski, K. et al, "DTPMPA: polyamino polyphosphonic acid and its use in Paper Processes, Part 1: The chemistry of Pulp Bleaching with DTPMPA and Its Impact on Fines Retention", Tappi J., 71(6):171-174 (1988)).
Hemp and straw fibers are difficult to bleach. At a given peroxide dosage, the achievable brightness level is much lower with straw fibers than with wood fibers. In order to produce hemp and straw pulps of high brightness at economical levels of peroxide charge, it is important to choose suitable stabilizing systems for peroxide bleaching liquors as well as appropriate bleaching conditions which should be suited to the characteristics of hemp and straw fibers. It is widely recognized that the chemistry and morphology of hemp and straw, for example wheat straw, is different from those of wood. Wheat straw has a substantially different metal profile than wood--a lower content of transition metals and a higher content of magnesium, silicon and calcium. Also, wheat straw contains appreciable amounts of low-molecular-weight lignin and hemicelluloses, which are easily solubilized in alkaline medium. As a result, alkaline peroxide solutions are capable of substantially dissolving lignins from wheat straw (U.S. Pat. Nos. 4,649,113 and 4,957,599).
The above factors make it difficult to use alkaline peroxide for brightening hemp and wheat straw to high levels while preserving pulp yield by limiting the dissolution of its components.